Support WebSockets over HTTP/2 via RFC 8441 Extended CONNECT

[mkurz]

Problem

Pekko HTTP supports HTTP/2 and WebSockets, but WebSocket support currently appears to be limited to the classic HTTP/1.1 upgrade handshake.

RFC 8441 defines how to bootstrap WebSockets over HTTP/2 using Extended CONNECT:

• server advertises SETTINGS_ENABLE_CONNECT_PROTOCOL = 1
• client sends :method = CONNECT
• client sends :protocol = websocket
• request also includes :scheme, :path, and :authority
• server accepts with :status = 200
• WebSocket frames are then carried over the HTTP/2 stream

This would allow applications to serve HTTP/2 requests and WebSockets on the same HTTP/2 connection instead of falling back to a separate HTTP/1.1 WebSocket upgrade connection.

RFC: https://www.rfc-editor.org/rfc/rfc8441.html

Current Behavior

Pekko HTTP WebSocket handling is based on the HTTP/1.1 upgrade model:

• request validation expects Upgrade: websocket, Connection: Upgrade, Sec-WebSocket-Key, and Sec-WebSocket-Version
• accepted WebSockets return 101 Switching Protocols
• the existing WebSocketUpgrade API represents upgrading the connection to WebSocket

That matches RFC 6455 over HTTP/1.1, but not RFC 8441 over HTTP/2.

Expected Behavior

When HTTP/2 is enabled, Pekko HTTP should optionally support RFC 8441 WebSockets over HTTP/2.

At a high level this likely means:

• advertise SETTINGS_ENABLE_CONNECT_PROTOCOL = 1 when server-side support is enabled
• accept HTTP/2 Extended CONNECT requests with :protocol = websocket
• expose those requests to routing / low-level APIs similarly to existing WebSocket upgrade requests
• allow existing handleWebSocketMessages / WebSocketUpgrade style APIs to work where practical
• respond with HTTP/2 :status = 200 when the WebSocket is accepted
• connect the existing WebSocket frame/message stack to the HTTP/2 stream DATA flow
• map HTTP/2 stream end/reset/failure semantics sensibly to WebSocket close behavior

Motivation

Modern browsers support WebSockets over HTTP/2 via RFC 8441. Supporting this in Pekko HTTP would allow one server port and one HTTP/2 connection to handle both normal HTTP traffic and WebSockets.

This is useful for frameworks built on Pekko HTTP as well. For example, Play Framework can enable HTTP/2 on its Pekko HTTP backend, but WebSocket support still depends on the HTTP/1.1 upgrade path because Pekko HTTP does not currently expose RFC 8441 WebSocket support.

Notes

Netty has already added several low-level RFC 8441 building blocks, but still tracks high-level WebSocket-over-HTTP/2 support as an open feature request:

• Websocket over HTTP2 netty/netty#8011
• [Feature] WebSocket over HTTP/2 (h2+h2c) netty/netty#12352
• Add a new HTTP/2 pseudo header :protocol netty/netty#11192
• Add SETTINGS_ENABLE_CONNECT_PROTOCOL to the default HTTP/2 settings netty/netty#14995
• Add SETTINGS_ENABLE_CONNECT_PROTOCOL to the default HTTP/2 settings netty/netty#15312
• Add opt-in validation of mandatory pseudo-header fields for HTTP/2 netty/netty#16932

A third-party Netty implementation also exists:

• https://github.com/jauntsdn/netty-websocket-http2

Questions

• Would the Pekko HTTP project be open to RFC 8441 server-side support?
• Should this be implemented behind an explicit config flag, for example under pekko.http.server.websocket or pekko.http.server.http2?
• Should the existing WebSocketUpgrade API be reused for HTTP/2 Extended CONNECT, or should there be a separate API to distinguish HTTP/1.1 upgrade from HTTP/2 stream-based WebSocket bootstrap?